OBJECTIVE: The study investigated relationships between rapid cerebral hemodynamic modulation and attentional performance. Based on former results on complex cognitive functioning, a specific association between the first seconds of the hemodynamic response and performance was hypothesized. METHODS: Using transcranial Doppler sonography, blood flow velocities in the middle cerebral arteries of both hemispheres were recorded in 48 healthy subjects. The applied task comprised motor reactions on a visual stimulus which was preceded by an acoustic warning signal (interstimulus interval 5s). Task-induced hemodynamic changes were assessed second-by-second, and related to reaction time using analysis of variance and linear regression. RESULTS: A right dominant blood flow response was observed. Flow velocity increase in the middle fraction of the interstimulus interval, i.e. seconds 2 and 3 after the cuing signal, significantly correlated with reaction time. This was not the case for the very early and late components of the response. CONCLUSIONS: The results suggest a time-locked association between cerebral blood flow increase and attentional performance. This is in accordance with neurophysiological studies that revealed the closest relationship between brain perfusion and cortical activity during a similar time window. SIGNIFICANCE: The study supports the assumption of a specific, relatively early time interval in which relationships between cerebral blood flow and behavior become apparent.